Distillation separation is an unit operation widely applied to industrial processes in general, but consumes a large amount of energy. In the industrial field, therefore, studies have been conducted on an energy saving distillation systems. Such studies have brought about development of a heat integrated distillation column (hereinafter, HIDiC) as a distillation apparatus that save much energy.
As shown in FIG. 1, a basic system of the HIDiC has a structure where a rectifying section (high-pressure unit) and a stripping section (low-pressure unit) are provided such that they are separate from each other. Operation pressure of the rectifying section is set higher than that of the stripping section so that the operation temperature of the rectifying section can be higher than that of the stripping section. This enables a reduction in the amount of heat that is supplied to a reboiler because heat transfer occurs from the rectifying section to the stripping section when there is a heat-exchange surface therebetween. Heat of the rectifying section moves to the stripping section, and hence the amount of heat that is removed at a condenser can be reduced. As a result, high energy saving distillation apparatus can be achieved.
In order to put the concept of HIDiC to practical use, a distillation apparatus having a double-pipe structure as discussed in JP2004-16928A has been proposed (hereinafter, Patent Literature 1).
As shown in FIG. 2, this distillation apparatus, which includes a shell 51 and a plurality of tube units 52 installed in the shell 51, is formed by connecting each tube unit 52 to the shell 51 via upper tube sheet 53a and lower tube sheet 53b. 
Each tube unit 52 has a double-pipe structure. Inner pipe 54 of tube unit 52 is used as a rectifying section while outer pipe 55 surrounding an outer surface of inner pipe 54 is used as a stripping section. Packing (structured packing) 54a and 55a are placed along the inside of inner pipe 54, and between the outer pipe 55 and the inner pipe 54. Refer to FIG. 3 for tube unit 52. The plurality of tube units 52 are arranged so that outer walls 65 of outer pipes 55 can come into contact with each other.
Referring again to FIG. 2, liquid inlet of stripping section 56 to supply liquid feed to the outer pipe (stripping section) 55 and vapor outlet of stripping section 57 to discharge vapor from the outer pipe 55 are arranged in an upper part of the shell 51.
Above upper tube sheet 53a, channel 58a that communicates only with inner pipe (rectifying section) 54 is formed. An upper end of the outer pipe 55 is not connected to upper tube sheet 53a that is to be opened.
Liquid inlet of rectifying section 59 to supply liquid (reflux) to the inner pipe 54 and vapor outlet of rectifying section 60 to discharge vapor from the inner pipe 54 are arranged in upper channel 58a. 
Vapor inlet of stripping section 61 to supply vapor to the outer pipe 55 and liquid outlet of stripping section 62 to discharge liquid from the outer pipe 55 are arranged in a lower part of the shell 51.
Below lower tube sheet 53b, channel 58b that communicates with inner pipe 54 is formed. A lower end of outer pipe 55 is not connected to lower tube sheet 53b that is to be opened.
Vapor inlet of rectifying section 63 to supply vapor to the inner pipe 54 and liquid outlet of rectifying section 64 to discharge liquid from the inner pipe 54 are arranged in lower channel 58b. 
In the abovementioned distillation apparatus, liquid feed are supplied through liquid inlet of stripping section 56, and uniformly distributed to upper of outer pipes 55 of tube units 52. Among liquid feed supplied to the upper end of outer pipes 55, liquid descending from outer pipe 55 in being fractionated in the outer pipe 55, is supplied to the reboiler installed outside of column via liquid outlet of stripping section 62 and is reboiled. Vapor generated by the reboiler enters the column again from vapor inlet of stripping section 61. The vapor from vapor inlet of stripping section 61 is distributed to a lower surface of outer pipe 55 of each tube unit 52 and ascends in each outer pipe 55. The liquid that is left without being vaporized is discharged as a product of the column bottom.
The vapor ascending from the outer pipe 55 in being fractionated, flows to a compressor via vapor outlet of stripping section 57. The vapor passing through the compressor enters a rectifying section via vapor inlet of rectifying section 63. The vapor from vapor inlet of rectifying section 63 ascends from the lower surface of each inner pipe 54. The vapor ascending through inner pipe 54 in being fractionated exits from an upper surface of each inner pipe 54, and is supplied to the condenser outside of column via vapor outlet of rectifying section 60. The vapor from the rectifying section is totally or partially condensed by the condenser. When necessary, a part of the condensed liquid is supplied as reflux to inner pipe 54 via rectifying section liquid entrance 59, while the rest is discharged as a distillate product.
In this configuration, energy transfer occurs from the rectifying section (inner pipe 54) to the stripping section (outer pipe 55). Hence, an amount of heat that is supplied at the reboiler and an amount of heat that is removed at the condenser can be reduced, and energy efficiency can be very high.
However, the heat integrated distillation apparatus having the rectifying section and the stripping section formed into the double-pipe structures as discussed in Patent Literature 1 has the following problems 1) to 6).
1) The product cannot be obtained with side-cut stream. The side-cutting means that a product is withdrawn as an intermediate distillate product, during a distillation process until an end distillate is acquired from top of column.
In the distillation apparatus described in Patent Literature 1, the tube units of the double-pipe structures are arranged to come into contact with each other. Moreover, the outer pipes and the inner pipes are equipped with the structured packing As a result, no pipe arrangement can be formed to withdraw any intermediate distillate product from the inner pipe of each tube unit. Consequently, the structure disables side-cutting.
2) The feed stage where feed stream is provided cannot be optimized. This is because in the rectifying section and the stripping section formed into the double-pipe structures, packing heights thereof are equal, disabling free setting of the number of stages of the rectifying section and the stripping section.
3) The feed stage cannot be changed so as to meet the feed stream composition. This is because of the structure where free setting of the feeding stage position is disabled as described in 2).
4) Multi-feed stream (reception of a plurality of feed streams) cannot be dealt with. This is because of the structure where no feed stream can be supplied in the midway of the double-pipes as described in 1).
5) Maintenance of the apparatus is difficult. The tube units that use the structured packing are densely arranged to be adjacent to each other as described in 1). This disables complete access to the desired tube unit, and maintenance thereof cannot be carried out.
6) The heat exchanged rate between the rectifying section and the stripping section that uses double-pipes and in which there is no a degree of freedom in design for designing the heat transfer area, depends only on the temperature profile of the distillation column. Hence, in apparatus design, a degree of freedom in design of heat exchanged rate is small.
Q, the heat exchanged rate between the rectifying section and the stripping section, is represented by Q=U×A×ΔT, where U is an overall heat-transfer coefficient, A is a heat transfer area, and ΔT is a temperature difference between the rectifying section and the stripping section. In the HIDiC using the double-pipe structure, an inner pipe wall surface becomes a heat transfer area. This heat transfer area has a fixed value determined by a structure of the double-pipes. The overall heat-transfer coefficient also has a fixed value determined by the heat transfer structure and fluid physical properties involved in heat exchange. Thus, as can be understood from the heat exchanged rate formula, a heat exchanged rate on design specification can be changed based only on the temperature difference between the rectifying section and the stripping section, which is changed by the operating pressure of the rectifying section and the stripping section.